FIG. 1 shows a basic diagram of a state of the art LED driver circuit that is used, for example, in an image reading apparatus in conjunction with a CIS or CCD image sensor device. An LED array L1-L3 is coupled to switches S1-S3 and resistors R1-R3. The LED array L1-L3, switching circuit S1-S3, and resistors R1-R3 are arranged separately from the analogue front end (AFE) circuitry that is used to process image data received from the sensor device, such as a Photo Diode Array (PDA) for example.
In a colour image scanner, the LED array typically comprises Red (L1), Green (L2) and Blue (L3) LEDs. Light of each colour emitted from the respective LED illuminates a portion of a target object that is to be scanned, and the light reflected by the object is incident on a sensor device. Typically, the sensor device comprises an array of sensors arranged linearly as a line image sensor, each element of the sensor array comprising a photoelectric conversion element, such as a photodiode and a capacitor for each pixel, which converts incident light into a current which is accumulated as a charge on the capacitor. The respective charges accumulated on the respective capacitors are converted into respective voltages that are then output from the sensor array (PDA).
The voltages output from the PDA are converted by an Analogue to Digital Converter (ADC) into digital signals for subsequent processing during generation of the image of the target object being scanned.
The scanning of an image is usually performed using a line scanning operation. For colour images, each line is scanned by the Red, Green and Blue light sources. That is, the Red LED L1 is turned on to read one line in a scanning direction, thereby obtaining the Red component of that line. The Green LED L2 is then turned on to obtain the Green component of that line, followed by the Blue LED L3 being turned on to obtain the Blue component of that line. The LED array and sensor array are then typically moved on a carriage mechanism to align with the next line on the target object. Each LED L1-L3 is turned on by switching on the respective switch S1-S3, using respective switch control signals CS1-CS3 received from a switch controller logic circuit SC.
While one line is being scanned, image data received from the sensor array (PDA) relating to a previous line scan is read out serially and processed by the ADC.
The current flowing through each LED is defined by the current-voltage characteristics of the LED, by the resistance of the series resistor and by the voltage applied from the power supply, PSU, (the on-resistance of the switch usually being negligible).
In addition to each LED passing a respective constant current during illumination of the image, it is also known to use Pulse Width Modulation (PWM) control signals for controlling the illumination of each LED, such that the illumination or intensity of the LED can be controlled by controlling the duty-cycle and/or frequency of the PWM control signals. The current though the LED may be subject to wide variation due to tolerances of the power supply voltage and the (temperature-dependent) I-V characteristic of the LEDs.
The LED driver circuitry is separated from the analogue processing circuitry due to the fact that, if the switching circuitry were integrated on the same Integrated Circuit (IC) as the analogue processing circuitry, i.e. AFE, the switching circuitry would interfere with the accuracy of the ADC that is used during an image scanning process.
Such interference is caused by switching transients which occur when switching from one LED to another (e.g. from Red to Green), and/or when pulse width modulating a specific one of the LEDs (e.g. switching on and off a LED using PWM control signals). The transient signals are coupled primarily through the supply or return ground paths, and cause inaccuracies when performing an analogue to digital conversion. For example, when the analogue processing circuitry is processing data obtained from a previous scan line, transient signals generated during a current scan line will adversely affect such processing. The effect of transient current spikes is exacerbated by the inductance of bond wires connecting the integrated circuit substrate to the external (PCB) ground. Similar effects occur in both monolithic integrated circuits and, to a lesser extent, in hybrid integrated circuits.
FIGS. 2a-2d show the switch control signals CS1-CS3 and the supply current IS drawn from the power supply PSU in a “constant current” (i.e. non-PWM) mode. The ground current is substantially equal to the supply current.
FIG. 2d illustrates how the supply current IS drops and rises during a switchover from one LED to another in the circuit of FIG. 1, thereby causing unwanted transient signals that can affect the analogue to digital data processing.
FIGS. 2e-2h show the switch control signals CS1-CS3 and the supply current IS drawn from the power supply in PWM mode.
FIG. 2h illustrates how the supply current (IS) drops and rises during PWM control, and when changing from one LED to another in the circuit of FIG. 1, thereby causing unwanted transient signals that can affect the analogue-to-digital data processing.
It is an aim of the present invention to provide a driver apparatus in the form of an integrated circuit for driving a light source array that does not suffer from the disadvantages mentioned above, namely lack of accuracy in setting the LED currents and/or transients in the ground and supply currents which make it difficult to integrate the switches and the AFE on the same substrate or integrated circuit.